1. Field of the Invention
The present invention relates to a semiconductor device which includes a memory cell portion including a memory cell gate and selection gate and a peripheral circuit portion including a peripheral gate, and a manufacturing method of the semiconductor device.
2. Description of the Related Art
An electrically erasable programmable read-only memory (EEPROM) which electrically writes/erases data has heretofore been known as a semiconductor memory. In the EEPROM, memory cells are arranged in intersections of rows and columns intersecting with one another to constitute a memory cell array. In the memory cell, a MOS transistor including a stacked gate structure is used in which floating and control gates are stacked.
In the EEPROM, at a data write/erase operation time, an operation is performed which comprises: applying a strong electric field between the control gate and a channel; passing a tunnel current through a gate insulation film; and implanting or removing charges with respect to the floating gate. In this operation, when a tunnel current flows in the vicinity of the gate insulation film, an electric stress is applied to the gate insulation film.
Moreover, it is generally known that there are a large number of traps with respect to the charges in a silicon nitride film. Particularly when the charges are captured in the traps in the silicon nitride film covering the surface of a source/drain diffusion layer, the diffusion layer in the vicinity of the surface of a substrate is depleted. As a result, a parasitic resistance of the source/drain increases, and drop of on-current of the transistor is caused.
Moreover, when the charges are trapped in the silicon nitride film in the vicinity of the gate insulation film, deterioration of electric characteristics occurs such as fluctuation of a threshold voltage of the transistor, and drop of withstanding voltage of a silicon oxide film. That is, when a material having high dielectric constant exists between the gates of the memory cell transistors disposed at a small interval, electrons are trapped. This raises a problem that the value of even a transistor disposed adjacent to the transistor with voltage applied thereto rises.
Particularly in a NAND flash memory, a large number of, such as 16 or 32, memory cell transistors are connected in series and are arranged close to one another. Therefore, when miniaturization advances, a voltage change of a specific memory cell transistor exerts an influence onto another memory cell transistor disposed adjacent to the specific transistor, and there is possibility of an erroneous operation or a problem in reliability.
When there is a film containing a large hydrogen content in the vicinity of the gate insulation film, hydrogen is taken into the silicon oxide film and structure defects such as Si—H coupling are easily generated in an interface with a silicon substrate.
More specifically, a portion of SiO2 bonds is replaced with H, thereby making free bonds which serve as trap centers.
When the above-mentioned Si—H coupling is cut by an electric stress, and the like, a cut portion functions as a trap with respect to the charge. Particularly when this trap is generated in the silicon oxide film as the gate insulation film, or a post oxide film in the vicinity of the gate insulation film, the deterioration of the electric characteristics such as fluctuation of the threshold voltage of the transistor and drop of the silicon oxide film withstanding voltage are caused.
Moreover, the charges are captured in the traps of the post oxide film with which the surface of the source/drain diffusion layer is covered. Then, the diffusion layer in the vicinity of the substrate surface is depleted. As a result, the parasitic resistance of the source/drain increases, and drop of the on current of the transistor is sometimes caused.
This problem becomes particularly remarkable, when a gate length is smaller than about 0.2 μm. That is, when the silicon oxide film, post oxide film, or silicon nitride film in which the traps are generated in the vicinity of the gate insulation film occupy a large ratio in the whole gate, this problem becomes remarkable.
The silicon nitride film is necessary for selective etching in forming a contact hole. On the other hand, since adverse influence of the silicon nitride film is seen with respect to the electric characteristics, it is difficult to enhance both yield and reliability of the semiconductor device.
To solve the problem, in Jap. Pat. Appln. KOKAI No. 2002-280463 as a prior application, when a contact hole is formed after gate processing, under a second insulation film (silicon nitride film) as an etching stopper, another first insulation film is disposed. This first insulation film is disposed to fill between gate electrodes of the memory cell transistor. Thereby, the influence of hydrogen in the second insulation film or charges captured in the second insulation film onto the electric characteristics of a device can be reduced.
Particularly in a memory cell portion, the first insulation film fills between the gate electrodes, and the second insulation film does not exist in the vicinity of the gate insulation film of the transistor. Therefore, the characteristics of the memory cell transistor can be prevented from being deteriorated, and reliability enhancement of the device results. That is, when the silicon oxide film is disposed between the gates of the memory cell transistor, the charges can be prevented from being trapped in the film having high dielectric constant between the gates.
However, in this structure, the first and second insulation films are also formed simultaneously with respect to a peripheral transistor. For the peripheral transistor, different from the memory cell, it is necessary to form an LDD structure as a hot electron countermeasure, or suppress a short channel effect by diffusion of impurities. To satisfy the above-described demand, it is a problem that the insulation film which is thick enough to fill between the memory cell transistors are also formed in the peripheral transistor. That is, when a gate sidewall insulation film is thick, the source/drain diffusion layer is offset from the gate, and the characteristics deterioration of the peripheral transistor are brought.
Therefore, there has been a demand for realization of a structure of a semiconductor device and manufacturing method of the device in which a peripheral transistor can efficiently be formed simultaneously with a high-reliability memory cell, and high reliability and high yield can be achieved.